Flake finishes (finishes having a general background of high-gloss paint embedded in which are nearly-flat reflecting flakes) are commonly used (for example, in the automotive industry) for their aesthetic effect. The appearance of a flake finish changes dramatically with a change of viewpoint because of the highly directional reflective characteristics of the embedded flakes--the finish appears to "glitter" and sparkle as the eye catches the reflections of different flakes. Unfortunately, no objective qualitative analysis technique has been available in the past which is capable of accurately measuring the optical characteristics of a flake finish and relating the measurement results to the physical characteristics of the finish. The highly prized optical characteristics of flake finishes have thus made flake finish quality control, finish matching and the like extremely difficult tasks requiring skill, guesswork and much experience.
The following documents disclose various techniques for characterizing the optical properties of flake finishes:
______________________________________ Alman U.S. Pat. No. 4,479,718 McCarty U.S. Pat. No. 3,601,589 McCarty et al U.S. Pat. No. 3,916,168 Armstrong. Jr., et al U.S. Pat. No. 3,690,771 Ward U.S. Pat. No. 3,060,790 Schreckendgust U.S. Pat. No. 3,389,265 Ishak U.S. Pat. No. 3,806,256 Anati et al U.S. Pat. No. 3,814,932 Holschlag et al U.S. Pat. No. 3,935,436 Kotera et al U.S. Pat. No. 4,090,243 Failes U.S. Pat. No. 4,165,180 Polcyn et al U.S. Pat. No. 4,191,940 Lawrence et al U.S. Pat. No. 4,278,538 Yuasa U.S. Pat. No. 4,402,611 Orchard U.S. Pat. No. 4,572,672 Van Dyk et al U.S. Pat. No. 3,708,233 Armstrong, Jr. et al U.S. Pat. No. 3,712,745 Provder et al U.S. Pat. No. 4,344,709 European Patent Publication No. 0 079 517 ______________________________________
U.S. Pat. No. 4,479,718 to Alman (1984) teaches based on empirical data that colorimetric measurements at three angles (110.degree., 45.degree. and 15.degree. from the direction of a specular beam) are necessary and sufficient to characterize the optical properties of a flake finish. See also U.S. Pat. No. 4,572,672 to Orchard (1986). Thus, the prior art teaches performing colorimetric measurements of light reflected by a flake finish at three different angles to permit matching of different finishes.
Unfortunately, tri-stimulus (Lab) values produced by goniometric analysis have only limited usefulness in matching paint finishes, for they provide little guidance as to what steps might be taken to make mismatched flake finishes match better.
Tri-stimulus values indicate the color appearance and, more importantly to most applications, the magnitude and direction of the difference in color appearance between a standard finish and a finish under measurement. Although differences in tri-stimulus values obtained from measurement of different flake finishes indicate the magnitude of differences in appearance of the finishes, trial and error (along with much experience and skill) is necessary to interpret tri-stimulus difference values and take appropriate corrective action.
A flake finish analysis technique which provides specific guidance concerning what physical characteristics of the finish under test should be altered to achieve closer matching with a standard finish would be extremely valuable.